The present invention relates generally to a printer having a plurality of selectable type rings or character wheels, and, in particular, to a small-sized printer having an improved type selection mechanism which controls at least two type rings by a lever actuated by an electromagnet.
Printers of the type under discussion having a plurality of type rings controlled by levers and electromagnets are disclosed in U.S. patent application Ser. No. 06/344,133, filed Jan. 29, 1982 assigned to the same assignees as the present application and Japanese laid-open patent publication No. 54-16228. In application Ser. No. 06/344,133, each type ring includes its own pawl lever, with two pawl levers being directly actuated by a single electromagnet.
Japanese laid-open patent publication no. 54-16228 discloses a printer mechanism having fewer electromagnets than type rings. According to this publication, in order for a trigger lever to engage a ratchet wheel disposed on the side of a type ring to stop the rotation of the type ring, the trigger lever must be driven by an armature lever and will thereafter rotate due to inertial forces. When the trigger lever is subjected to disturbances such as variations in frictional loads on the trigger lever, the speed of rotation thereof varies resulting in the possibility of causing printer malfunction such as erroneous selection of a type element disposed on the type ring.
When a second type ring completes a first revolution and is about to make a second revolution, a second trigger lever goes over and beyond a stop on the type ring. In this regard, a cam for controlling the position of the trigger lever has three steps having different diameters. Due to mechanism limitations, however, such diameters cannot be increased with the result that a required degree of dimensional accuracy of the cam cannot be maintained, and the operation of the cam is rendered complex. A spring stopper must be displaced by the cam, and wear on the cam due to frictional contact with the spring stopper is significant. The armature lever is brought to a standby position when held against the spring stopper. A slight deformation of the spring stopper would vary the gap between the armature lever and the attracting portion of the electromagnet, a condition which would render electromagnet performance unstable and cause malfunction of the printer.
When selecting a first type ring, two trigger levers are required to be driven by the electromagnet. The second type ring can be selected by driving a single trigger lever when the first type ring has been selected, and by driving two trigger levers when the first type ring has not been selected. When the two trigger levers are driven, due to frictional contact with the stop on the type ring, the electromagnet is subjected to an increased load which tends to vary greatly. An attempt to construct an electromagnet having a capacity large enough to bear the required load would require an increased expenditure of cost and result in design problems. Accordingly, a printer which requires a single electromagnet which controls a single lever which operates at least two type rings, which is efficient and stable in operation and which avoids the deficiencies of the prior art, is desired.